| Issue |
143669
|
| Summary |
[Clang] Alias template argument deduction is enabled in SFINAE contexts in C++17 mode
|
| Labels |
clang:frontend,
diverges-from:msvc
|
| Assignees |
|
| Reporter |
frederick-vs-ja
|
After #77890 (implementing [P1814R0](https://wg21.link/p1814r0)) and possibly #89358, Clang starts to report CTAD for alias template can be well-formed in immediate contexts even in C++17.
As a result, the following example starts to be rejected in C++17 mode since Clang 19. [Godbolt link](https://godbolt.org/z/nYnbs8nd9).
```C++
template <class T>
T quick_retval();
template <class, template<class> class Tmpl, class...Args>
constexpr bool CanCtadFromImpl = false;
template <template<class> class Tmpl, class...Args>
constexpr bool CanCtadFromImpl<
decltype(static_cast<void>(Tmpl(::quick_retval<Args>()...))), Tmpl, Args...> = true;
template<template<class> class Tmpl, class...Args>
constexpr bool CanCtadFrom = CanCtadFromImpl<void, Tmpl, Args...>;
template <class T>
struct Foo {
explicit Foo(T);
};
template <class T>
using AliasedFoo = Foo<T>;
static_assert(CanCtadFrom<Foo, int>);
static_assert(!CanCtadFrom<AliasedFoo, int>);
```
If I understand correctly, `CanCtadFrom<AliasedFoo, int>` should be `false` in C++17 and `true` in C++20 or later.
GCC and EDG seem to be consistent with Clang (i.e. making `CanCtadFrom<AliasedFoo, int>` `true` in C++17 mode), while MSVC makes `CanCtadFrom<AliasedFoo, int>` `false` in C++17 mode and `true` later.
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